1. Field of the Invention
The present invention relates to the preparation of vinyl chloride by ultrapyrolysis of 1,2-dichloroethane, and, more especially, to such ultrapyrolysis whereby the 1,2-dichloroethane is heated to an elevated temperature as rapidly as possible and, after approximately 0.1 to 0.5 second, the medium of reaction is advantageously quenched to block any further reaction.
2. Description of the Prior Art
Pyrolysis of 1,2-dichloroethane (D12) in the gaseous phase is the most widely used industrial process for producing vinyl chloride, the basic starting material for PVC. This process is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A6, pages 287-289 (1986).
According to this process, 1,2-dichloroethane is heated in a tubular oven, on the side of the tubes, to 500.degree. C.; the degree of conversion is 50% to 60%, the selectivity 95% to 99%, and the residence time ranges from 10 to 20 seconds.
If it is desired to increase the conversion, there is a risk of coke depositing inside the tubes. Avoiding such coke deposits by dilution of the 1,2-dichloroethane with hydrochloric acid prior to the pyrolysis is described in European Patent EP-195,719, assigned to the assignee hereof. Nonetheless, this particular pyrolysis process requires very heavy and expensive apparatus.
The article by Paouschkin and Charnaia, Neftekhimia, Vol. 10, part 4, pages 583-585 (1970), describes the pyrolysis of 1,2-dichloroethane in the presence of steam at a concentration of 2.5 parts of water per 1 part of 1,2-dichloroethane. The pyrolysis is carried out in a reactor, the walls of which are covered by carbon deposits, at a temperature ranging from 600.degree. to 850.degree. C. It is described that "once having passed through the evaporator at a temperature of 600.degree. C., the steam and the 1,2-dichloroethane at a given rate were brought into the reactor." Then indicated is a volumetric rate of 0.73 hour.sup.-1 ensuring a contact time equal to 0.003 seconds, which apparently is inconsistent. It is not seen how the steam/1,2-dichloroethane mixture can be conveyed through an evaporator at 600.degree. C. and be heated to a temperature of from 600.degree. to 850.degree. C., all in 0.003 second. Moreover, the results demonstrate a large amount of acetylene byproduct. It is also very problematical to carry out the pyrolysis on an industrial scale in the presence of water, the separation from HCl being difficult.
European Patent Application EP-281,218 describes a hydrocarbon cracking process for the production of olefins in which a stream of particles, previously heated to a temperature of from 926.degree. C. to 1,648.degree. C., is contacted for 10 to 100 milliseconds (preferably 20 to 50 milliseconds), with a stream of naphtha or heavy oils, themselves heated beforehand to a temperature of from 260.degree. C. to 690.degree. C., into which steam is also injected (steam cracking). The ratio by weight of the particles to the stream of naphtha to be cracked ranges from 5 to 200, namely, the lowest temperature attained by the naphtha is, in the event of the particles at 926.degree. C., naphtha preheated to 260.degree. C., and the particles/naphtha ratio having a value of 5, at approximately 800.degree. C.
The highest temperature attained by the naphtha is, in the event of particles at 1,648.degree. C., of the naphtha preheated to 690.degree. C. and the particles/naphtha ratio having a value of 200, at approximately 1,648.degree. C. On page 24, lines 56-58, this technique is suggested for pyrolyzing 1,2-dichloroethane, at a temperature of from 800.degree. to 1,648.degree. C., preferably for 0.02 to 0.05 second, and in the presence of water.
If the kinetic equation of D. H. R. Barton (Journal of Chemical Society, page 148 (1949)) is considered, it is determined that, at 570.degree. C. for 0.160 s in a reactor with a surface/volume ratio of 3.6 cm.sup.-1, 2.4% of conversion of the 1,2-dichloroethane is obtained, the 1,2-dichloroethane having been diluted beforehand with nitrogen, nitrogen/1,2-dichloroethane molar ratio =10.
With a contact time of 0.050 s, a conversion of 0 (zero) % is obtained.
Moreover, it is known that, by carrying out the reaction at high temperatures, serious risk exists of producing substantial acetylene byproduct.